Ions in Aqueous Solutions
and Colligative Properties
Chapter 14
Dissociation
The separation of ions that occurs when an ionic compound
dissolves is called dissociation.
NaCl(s)
𝐻 2𝑂
CaCl2(s)
Na+(aq) + Cl-(aq)
𝐻 2𝑂
Ca2+(aq) + 2Cl-(aq)
Dissociation
Dissociation
Write the equation for the dissociation of magnesium chlorate in
water.
How many moles of ions are produced for every 1 mol of
magnesium chlorate?
How many moles of ions are produced for every 38.3 g of
magnesium chlorate that dissolve?
Ionization
 Ionization differs slightly from dissociation.
 Ions are formed from solute molecules where none existed
previously.
HCl(aq)
𝐻 2𝑂
HCl(aq)+ H2O(l)
H+(aq) + Cl-(aq)
𝐻2𝑂
H3O+(aq) + Cl-(aq)
Strong and Weak Electrolytes
Substances that produce solutions that are capable of conducting an
electric current are called electrolytes.
Substance must produce ions when dissolved
 Salts
 Acids
 Bases
Strong electrolyte – compound whose solution conducts strongly
NaCl(s)
HCl(aq)
𝐻2𝑂
𝐻2𝑂
Na+(aq) + Cl-(aq)
H+(aq) + Cl-(aq)
Weak electrolyte – compound whose solution conducts weakly
CH3COOH(aq)
𝐻2𝑂
H+(aq) + CH3COO-(aq)
Colligative Properties
Properties that depend on the concentration of solute particles but
not their identity
Vapor Pressure Lowering
 Lower the vapor pressures of solutions in comparison to the
pure solvent
 Solute particles get in the way
Colligative Properties
Boiling Point Elevation
 Lower the vapor pressures of solutions in comparison to the
pure solvent
 Solution must be raised to a higher temperature in order for
EVP to equal atmospheric pressure
Freezing Point Depression
 Solution must be cooled to a lower temperature in order for it
to freeze
Both BP elevation and FP depression are proportional to the
number of solute particles dissolved in solution.
Colligative Properties
Colligative Properties
Boiling Point Elevation
∆TBP = KBP · m
where KBP for H2O = 0.512
Freezing Point Depression
∆TFP = KFP · m
where KFP for H2O = 1.86
Colligative Properties
Colligative Properties
How much salt would you have to
add to raise the boiling point of
water 5°C?
∆TBP = KBP · m
Colligative Properties
Osmotic Pressure
Colligative Properties
Desalination